1. Field of the Invention
The present invention relates to an apparatus for communicating with an RFID (Radio Frequency Identification) tag through a radio wave (hereinafter, referred to as tag communication apparatus), a control method for a tag communication apparatus, a computer readable medium for tag communication control and a tag communication control system.
2. Description of the Related Art
In recent years, the use of an RFID (Radio Frequency Identification) tag (radio rag) has been progressively spread. The radio tag is anticipated especially in a field of physical distribution as an alternative of a bar code. Thus, the radio tag is expected to be widely used in near future.
Currently, a frequency band directed to the radio tag includes a 13.56 MHz band, what is called an UHF band of approximately from 800 MHz to 960 MHz, a 2.45 GHz band or the like. The radio wave of the UHF band or the 2.45 GHz band of these bands has an advantage that a communication distance can be easily extended longer compared to that of the radio wave of the 13.56 MHz band. Further, the radio wave of the UHF band advantageously goes round to shadows more easily compared to the radio wave of the 2.4 GHz. Accordingly, the radio tag and a reader/writer using the radio wave of the UHF band have been progressively developed.
When the radio wave of the UHF band is employed, a distance in which the reader/writer can communicate with the radio tag can be extended from several ten centimeters to about several meters, as compared with a case in which the radio wave of the 13.56 MHz band which is used mainly currently is employed. Therefore, when the radio wave of the UHF band is employed, a communication area as a space region in which the reader/writer can communicate with the radio tag can be extended to a relatively wide range. (Refer to JP-A-2002-198722, JP-A-9-5431, “View of GPS technology”, Journal of Institute of Electronics, Information, and Communication Engineers, B, Vol. J84-B, No. 12, pp. 2082-2091, December 2001, “Highly accurate position measuring technique using portable telephone that can use positional information service even indoors” http://www.hitachi.co.jp/Sp/TJ/2001/hrnjan01/hrn0111j.htm, “Adaptive signal processing by array antenna”, Chapter 9, High resolution estimation of incoming direction by array antenna, Nobuyoshi Kikuma, Science Press, Inc., issued on Nov. 25, 1998.)
As described above, when the distance in which the reader/writer can communicate with the radio tag is set to about several meters, the communication area is widened. Thus, the number of the radio tags present in the communication area is undesirably increased more than the required number. Accordingly, various methods may be considered for limiting the communication area to a specific space region.
FIG. 15A shows an example in which beam forming areas are overlapped to control a communication area. In this example, a plurality of antennas 51 whose directivity is narrowed by a beam forming operation is arranged. Thus, a space region obtained by overlapping communication areas 52 respectively formed by each of the antennas 51 provided in the reader/writer is set as the communication area by the reader/writer.
FIG. 15B shows an example in which a communication area is controlled by a beam scanning operation. In this example, an antenna 51 whose directivity is narrowed is used. A predetermined space region is scanned by sequentially changing a directive direction of the antenna 51 within a predetermined range. Further, in this example, two antennas 51 that scan different space regions respectively are provided. A space region obtained by overlapping each of the scanned areas is set as a communication area by the reader/writer.
In the above-described two examples, since the antennas 51 having a high directivity are employed, following merits are obtained as compared with a case in which the antennas 51 having a low directivity are used. Firstly, since the communication area 52 covered by the single antenna 51 is narrow, the generation of the collision of the communication can be suppressed and the reliability of the communication can be improved. The collision of the communication means that the collision of the communication arises between the antenna 51 and the number of radio tags, since a number of radio tags are present in the communication area 52 corresponding to the certain antenna 51 at the same time. When the collision of the communication arises, the reliability of the communication is lowered.
Further, when the antenna having the low directivity is used, an incommunicable region where the reader/writer cannot communicate with the radio tag is generated within the communication area, which is due to an interference between a direct wave from the reader/writer and a reflected wave which is reflected by a floor surface or a wall surface. However, when the antenna 51 having the high directivity is used, such an inconvenience can be eliminated.
However, the above-described examples have the following problems. A first problem is that an influence (incommunicable region) due to a multipath arises. FIG. 16A shows a state that the multipath is generated. When a reflector 54 exists in the direction of the directivity of the antenna 51, a beam outputted from the antenna 51 is reflected by the reflector 54. Thus, the beam is also irradiated to a space region located outside the direction of the directivity of the antenna 51. Accordingly, communication with a radio tag located in a space region other than an estimated communication area is undesirably carried out.
In order to prevent this area distortion, for every place in which the reader/writer is installed, an environment where the reader/writer is installed needs to be tuned, or a radio wave cut-off configuration needs to be provided so as not to be influenced by the multipath. For instance, as shown in FIG. 16B, a radio wave absorber is provided on a surface opposed to the antenna 51 in the communication area in order to prevent the multipath.
Further, a second problem is the structural size of the antenna 51 is enlarged. When the communication area is controlled by overlapping the beam forming areas or performing the beam scanning operation as in the above-described examples, the directivity of the antenna 51 needs to be relatively enhanced. The antenna 51 is configured with, for instance, a patch antenna or an array antenna. In order to strengthen the directivity, the number of antenna elements 51a provided in the antenna 51 needs to be increased. FIG. 17A shows a state of the antenna elements 51a provided in the antenna 51 when the directivity is relatively low, and FIG. 17B shows a state of the antenna elements 51a provided in the antenna 51 when the directivity is relatively high, respectively.
Further, a third problem is that when a beam scanning operation is carried out, in order to enhance the resolution in a scanning direction, a control of a voltage applied to the antenna elements 51a and the phase thereof becomes complicated. FIG. 18 shows a state when the scanning direction of the antenna 51 is changed. As shown in FIG. 18, the antenna elements 51a are respectively provided with voltage controlling phase shifters 51b. In order to change the scanning direction, the voltage applied to the antenna elements 51a respectively corresponding to the voltage controlling phase shifters 51b and the phase of the voltage need to be controlled. Here, in order to enhance the resolution in changing the scanning direction, the voltage controlling phase shifters 51b respectively need to be controlled more finely. Thus, calculation for the control becomes more complicated, and each of the voltage controlling phase shifters 51b needs to be highly accurate.
On the other hand, a method may be considered for limiting the communication area by specifying the position of a radio tag performing a communication, and determining whether or not the position of the radio tag is located in a predetermined space region. Here, in “View of GPS technology”, Journal of Institute of Electronics, Information, and Communication Engineers, B, Vol. J84-B, No. 12, pp. 2082-2091, December 2001, a technique that specifies the position of a GPS receiver by using a GPS is disclosed. In this technique, the GPS receiver measures the incoming time of radio wave from a plurality of GPS satellites to calculate distances between each of the GPS satellites and the GPS receiver. Then, the position is specified based on the calculated distances.
Further, in “Highly accurate position measuring technique using portable telephone that can use positional information service even indoors” http://www.hitachi.co.jp/Sp/TJ/2001/hrnjan01/hrn0111 j.htm, a technique is disclosed that the same system as that of the position detection by the GPS is applied to a portable telephone to specify the position of the portable telephone. In this technique, a base station in a portable telephone network plays the role of the GPS satellite in GPS to measure the position.
The above-described techniques may be applied to a specifying operation of the position of the radio tag so as to limit the communication area. However, in this case, following problems arise. Initially, in the radio tag side, for instance, a complicated circuit for measuring distances to the plurality of the antennas is required. For instance, to measure the distance between the antenna and the radio tag, timekeeping section for measuring the transmission time of the radio wave is required. To measure the transmission time of the radio wave located within a distance of about several meters or smaller, extremely highly accurate timekeeping operation needs to be carried out. Further, a synchronization of time with each of the antennas needs to be extremely highly accurately carried out. Since the radio tag basically and preferably has features such as low cost, compact form and low power consumption, it is not preferable to mount such a circuit having high functions on the radio tag.